In prior art it has been proposed to use phase contrast in x-ray imaging to increase signal-to-noise ratio (SNR) in e.g. mammographic applications. Medical x-ray imaging is often limited by small contrast differences and high noise caused by tight dose restraints. This is particularly true for mammography where low contrast tumors constitute a major detection target, and a large number of tumors are missed or misdiagnosed due to difficulties in detection. The use of phase contrast imaging in medical applications have shown promising in order to increase SNR, since the phase shift in soft tissue is in many cases substantially larger than the absorption.
International patent application WO 2008/006470 A1 describes the use of interferometers for x-rays wherein x-ray images can be acquired from a scanned object. The set up herein comprises means for evaluating intensities on a pixel 105p basis in order to identify characteristic of the object 108 by characterizing each pixel 105p as being for instance phase contrast or absorption contrast dominated. In one application concerning the investigation of luggage on a moving conveyor belt, a set-up comprising an array of line detector 105s and a number of sub-gratings are arranged between the object 108 and the line detector 105s wherein each of the sub-gratings are shifted in their position perpendicular to the grating lines. In this manner, luggage to be investigated is moved along a direction perpendicular to the grating lines during a scan, wherein one scan movement is required to acquire phase contrast and absorption contrast data.
There are a number of disadvantages with the art presented above. First of all, the solution requires the manufacturing of physically long sub-gratings G1n and G2n, which is consumes resources in terms of cost and time.
Another drawback is that the proposed set up if directly implemented in a mammography application is likely to induce errors in the phase detection due to the direction of the scan vs the direction of phase contrast detection. When a stationary object 108 such as a breast is analyzed, the set up needs to be moved in a scan direction in relation to the object 108 to create an x-ray image of the object, not the other way around as in WO 2008/006470 A1. In WO 2008/006470 A1, the scan direction is set to be perpendicular to the grating lines of the sub-gratings, and hence perpendicular to the interference fringes 163 to be detected. It is well known that any system, including scanning systems, generally introduce more disturbances in the scan direction since this is the direction of change. Another source of disturbance is the gravity itself on a moving scan arm 103, since a gravity component of the scan arm 103 will induce a torque on a detector 105/analyzer grating 162 relative to the beam 122 splitter. Hence, a small shift between the analyzer grating 162 and the beam 122 splitter due to gravitation will further induce errors in the phase detection. In conclusion, in order to reduce these disturbances a set up as in the prior art needs have very high requirements on scan precision, which will make the manufacturing of such products more costly as well as time-consuming.
Yet another drawback of the prior art is that the full potential of using phase contrast, especially sought for in mammographic applications, is not utilized. One of the main advantages of using phase contrast imaging is the reduced noise at high spatial frequencies, i.e. an improved ability to detect small features. In a scanning system, spatial resolution is generally lower in the scan direction since continuous read out is most often implemented.